This invention relates to thermoplastic compositions comprising blends of acrylic sequential graft copolymers and styrene-acrylonitrile copolymers having reduced mold shrinkage and increased surface hardness characteristics and to a method for improving the mold shrinkage and surface hardness of acrylic sequential graft copolymer thermoplastic elastomer.
The term "thermoplastic elastomer" has generally been applied to elastomers that can be readily processed and reprocessed by conventional melt processing equipment by virtue of the fact that such elastomers are not cured or vulcanized. The reprocessability of these elastomers compared with conventional cured or thermoset rubbers results in a great reduction in loss due to scrap, with consequent economic benefits for the processor. A variety of such materials have been introduced in recent years such as thermoplastic polyesters, styrene block copolymers, and thermoplastic olefin-rubber blends. Typical of such materials are the styrene-butadiene-styrene block copolymers sold as Kraton brand elastomers by the Shell Chemical Co. and the Hytrel brand polyester elastomers sold by DuPont. Many of these elastomers have found wide application in consumer goods such as in shoe soling formulations and the like, as well as in such industrial applications as wire coating, hose and tubing, electrical connectors and automotive parts.
High rubber content graft copolymers, by which is meant graft polymers prepared by graft-polymerizing rigid monomers in the presence of rubbery polymeric substrates and comprising greater than 50 wt% rubbery substrate polymer, are widely used as impact modifiers for thermoplastics such as styrene resins, styrene-acrylonitrile (SAN) resins, PVC resins and the like. Typical of such materials are graft copolymers of styrene and acrylonitrile and optionally methylmethacrylate on diene rubber substrates such as the Blendex brand modifiers sold by Borg-Warner Chemicals, Inc. and graft copolymers of methacrylates on acrylic rubber substrates and of methacrylates on diene rubber substrates sold as modifier resins by Rohm and Haas. A wide variety of such graft copolymers is readily available commercially. In general, rubbery graft copolymer impact modifiers are employed at levels of less than 30 wt% and often at less than 10 wt% in blends with rigid resins to provide high impact thermoplastics. Although often referred to as rubbery modifiers, these copolymers for the most part are not truly elastomeric and do not exhibit useful elastomeric properties without further modification and vulcanization. These materials therefore are not considered to be thermoplastic elastomers.
More recently, in U.S. patent application Ser. No. 560,360, filed Dec. 12, 1983, now U.S. Pat. No. 4,473,679 there were disclosed elastomeric compositions having a core-shell structure prepared by a sequential polymerization process, said compositions comprising (a) a rigid copolymer core formed of at least one rigid monovinylidene monomer and a copolymerizable functional monomer, (b) a rubbery copolymer outer layer formed of at least one rubbery arcylic monomer and a second copolymerizable functional monomer and (c) a transition layer, intermediate between the core and outer shell, formed of a copolymerized mixture of the core and shell components. These compositions are thermoplastic elastomers without being vulcanized or cured. Although quite useful for many applications, these compositions exhibit a fairly high degree of mold shrinkage, and may be somewhat soft for certain applications where surface hardness and stiffness are important. Formulations of these elastomers which exhibit reduced mold shrinkage and higher surface hardness would thus widen the potential end-uses for these resins.